Until VFX finishes their Asterisk support, and releases capability to the unsuspecting public, its still doable for about $200 of extra hardware. For those people that are sooo keen for VFX on their Asterisk box, this is a relatively economical way to do it, and the hardware should still fetch 75% of purchase price in 6months or a year when resold (assuming by then you will have VFX directly SIP trunking into your box.)

Problem with doing stuff like this is that you lose the ability to dial without massive delays while the dtmf does its thing, you lose knowing when the other end has hung up and you there is the chance of getting echo from the analog connection.